This project involved a cooperative research effort between the NIH and the Tokyo Institute of Technology. Using sonoelasticity, which combines ultrasonic imaging and mechanical vibrations, data on the elasticity of contracting skeletal muscles were collected. The first phase of the study involved equipment and software modifications. Although the protocol called for real-time measurements of muscle contractions, this was not possible with the existing hardware and, hence, static images of muscle contractions were obtained. A pulley apparatus was fabricated which allowed the application of a constant tension to the quadriceps muscle, and images were captured during static contractions. Medical data were collected from 10 healthy young male graduate student volunteers, and each subject was asked to maintain a series of 13-second contractions in each of three knee positions with applied weights of 0, 7.5 and 15 kg, during which sonoelastic data were collected. Preliminarily, an increase in muscle stiffness with increasing loads has been observed. The most striking finding has been of a tremendous heterogeneity in the stiffness of each muscle. Enhanced image processing methodologies are currently being applied in order to exclude potential artifactual causes of this variability. It is felt that the sonoelastic methodology has a major potential to contribute to the understanding of muscle contraction, particularly if the theoretical goal of near real-time data acquisition can be realized.